LABORATORY TESTS OF DEEP COMPOSITE WOOD-CONCRETE BEAM AND DECK SPECIMENS

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1

4
e
Conférence spécialisée en génie des structures
de la Société canadienne de génie civil

4
th
Structural Specialty Conference
of the Canadian Society for Civil Engineering

Montréal, Québec, Canada
5-8 juin 2002 /
June 5-8, 2002



LABORATORY TESTS OF DEEP COMPOSITE WOOD-CONCRETE

BEAM AND DECK SPECIMENS


R. Gutkowski
A
, J. Balogh
A
, C. Rogers
A
, R. SaRibeiro
B

A Department of Civil Engineering, Colorado State University, U.S.A.
B National Institute for Amazonian Research (INPA), Brazil


ABSTRACT:
Experimental and analytical studies of composite wood-concrete floor/deck systems have been
ongoing at Colorado State University. An innovative novel shear key/anchor detail for overlaying solid wood
floors in office buildings with a concrete layer, thus creating a composite floor, was adapted for this study. The
connection detail transfers bearing stress between and shear stress in the joined materials for interlayer force
transfer. Prior work was on a geometry intended for floors of moderately loaded systems (residences, offices
etc.). In continuation work tests are being conducted on deeper layered beam specimens, intended for heavier
loads (warehouses, bridges etc.). Laboratory studies include pull-out tests on the anchors; interlayer slip tests
on connection details; and pilot service, repeated load and creep tests of several layered beam specimens.
These will be followed by load tests of a full size deep floor/ deck specimen. This paper reports on the
outcome of early ramp tests of several specimens.



1.
INTRODUCTION


Solid reinforced concrete slab construction has some significant structural and cost shortcomings.
Tensile cracking occurs in the lower half or more of the slab and renders that portion of the concrete to be
structurally ineffective. The cracked concrete only holds the rebar in place. It also contributes to higher
deflection due to reduction in the moment of inertia. Exposed rebar can be subject to corrosion in
exposed environments, e.g in bridges. Recent research by the authors and others (Gutkowski et al. 1996,
1999, 2000) indicates that a layered wood-concrete floor system can be an effective option, even
preferable. A structurally effective solid wood layer could replace the ineffective concrete and expensive
steel reinforcement bar. The wood layer serves as formwork for the solid concrete layer during
construction. By effectively interconnecting the two layers to develop composite action, a structurally
efficient system results. The strong compressive strength of concrete is combined with the high tensile
stress in bending capacity of dimension lumber. An innovative method of interconnection has also been
developed to adequate partial composite action and other advantages.

Two important non-technical benefits of the mixed material construction are cost savings of replacing
non-renewable resource based concrete and steel with a managed renewable resource; and savings in
energy of material production and construction (Weber 1997, Winter 1998). Changes from concrete and


2
steel to more wood construction can substantially reduce energy requirements and carbon dioxide
emissions (Wegener et al. 1997, 1998). These realities and the outcomes of this study encourage
considering wood-concrete composites as a new application of dimension lumber in the U.S.A.


2.
SCOPE OF THE ONGOING RESEARCH


Past work by researchers at Colorado State University (CSU) has demonstrated the structural efficiency
of a notched shear/key anchored layered deck concept for effecting composite action in layered
wood-concrete residential and office floor construction (Gutkowski et al. 1999, 2000). Initially, a series of
full-scale load tests were conducted on interlayer connection specimens, preliminary layered beam
specimens and two full-scale wood-concrete floor specimens (Brown 1998, Brown et al 1998, Etournard
1998, Etournard et al. 1998, Thompson 1997). The connection tests served to assess interlayer load-slip
("slip" is the relative horizontal motion of the layers) behavior of the interlayer connection detail. The
beam tests were a cost efficient means of examining positioning of the notches and overall load
deflection behavior up to failure. Static and ramp load tests showed that a high degree of composite
action can be achieved. A basic computer model was successfully used to analytically simulate the
load-deflection response (Koike 1998, Koike et al. 1998). The interlayer connection details were included
as a spring having the load-displacement characteristics determined from the load-slip tests.

The main objective of the ongoing research program is to configure and load test full-scale wood-
concrete deck specimens sufficient in depth for use in short span highway bridges and short to
intermediate span pedestrian bridges. The interlayer connection detail has been altered to accommodate
the larger geometry and the higher loads. Dimensions were based on having determined a tentative
configuration of a longitudinal layered bridge for an actual roadway site. The thickness of the layers for
the beams were approximately determined by using the basic computer model to configure a bridge deck
sufficient to carry a highway bridge loading. The computer model was applied with some extrapolations of
spring properties obtained from the various past interlayer connection tests.

Preliminary to the construction and testing of the deeper deck specimen an experimental study of the
connection detail and beam specimens will be done to refine the deck specimen, if necessary. A modified
version of the shear key anchor detail will also be examined. A series of four layered deep beam
specimens have been configured to different widths but the same depth as the eventual bridge deck
specimen. To date, two have been load tested under ramp loading. Parallel tests of the interlayer
connection detail are planned. Beam specimens of various widths were constructed to examine the effect
of width on lateral load sharing. Subsequent to this work, a more rigorous analytical model will be
developed to more closely detail the complete deck specimen, including possible need for use of Mindlin
plate theory. Durability under creep and repetitive loads and extremes of temperature and humidity need
to be examined, particularly for possible applications in bridge decks. An extensive funded project was
recently initiated to examine these aspects.


3. INTERLAYER CONNECTION DETAIL

The notched shear key/anchor detail (see Fig. 1) was developed in Europe (Natterer et al.
1996,1997,1998).



3


Figure 1. Shear key/anchor detail.

To achieve adequate composite action the connections minimize interlayer slip. A dowel is glued into a
tapped pilot hole in the wood using a specified glue and procedure. The concrete is prevented from
bonding to the dowel by a plastic sleeve placed around the shank of the dowel. As the concrete cures,
the materials dry and shrink and separation of the surfaces can occur. Re-tightening of the dowel head
after curing brings the layers back into contact. The connection achieves low slip by transferring the
interlayer force through bearing stresses on the notch incline and horizontal shear in the concrete notch
key. The anchor is not stressed in shear and only resists the tendency for upward movement (uplift)
created by the bearing force.

For interior applications the shank of the dowel connector is kept entirely within the wood layer. This is
done for fire rating purposes and to not interfere with attaching ceiling surfacing. Normally, a ceiling
surface is not used because the exposed solid wood floor is visually attractive itself. In the case of
bridges, these constraints do not exist. Consequently, a modified detail is also included in the beam tests
(see Fig. 2).

Figure 2. Modified shear key/anchor detail.

The essential difference is the shank of the dowel extends completely through the wood layer. A metal
plate is attached to the end and bears against the underside of the wood layer. One potential advantage
is the elimination of the adhesive, which has some limitations and concerns identified in the past work
(Brown 1998, Brown et al. 1998).


4.
DESCRIPTION OF THE SPECIMENS AND CONDUCT OF THE EXPERIMENTAL TESTS


As stated earlier, the beam specimens were configured as a portion of the width of the layered
wood-concrete longitudinal bridge deck specimens. Figure 2 shows the main longitudinal dimensions of
the various specimens, notch locations and loading configuration. Figure 3 shows the end view of the
various specimens. For the specimen with glued anchors, as was done in the past work, the Hilti HIT HY
150 epoxy was used. The adoption of this adhesive was based on past withdrawal tests and load slip


4
tests of various adhesives initially considered feasible.

The wood used was Hem-Fir No. 2, surfaced dry, 38x235 mm (1.5”x9.25”) actual size dimension lumber.
An average 7.1% moisture content (MC) condition was determined. The layered wood beam sections
were 0.9-m (3’)-wide beams, composed of 24 vertical boards each. The wood members were laterally
nailed (nails were spaced at 406 mm (16”) and placed on two scattered rows along the length of the
beam) together with 101 mm (4”) long galvanized nails. Six notches spaced 990-mm (39”) on-center
along the length, were grooved across the top of the wood laminations (see Fig. 3 and 4). The shear
connectors required two 12.7 mm (1/2”) diameter pre-drilled holes with a depth of 101 mm (4”) in each
notch, spaced 457 mm (18”) on-center. Each dowel connector was set in place after applying the HIT-HY
150-epoxy adhesive to the hole. For the beam with the modified type connectors the holes were drilled
through the entire depth of the wood layer, and the connectors were held in place by screws in the bottom
plate. Shrinkage and temperature steel reinforcement for the concrete slab were placed to satisfy the
requirements of the ACI 318-99 concrete building code (ACI 1999) and are illustrated in Fig. 4.

The original Hilti dowel had a modulus of elasticity, E = 210,000 N/mm
2
(30,500 ksi), and yield strength, f
y
=
460 N/mm
2
(65.3 ksi). Using an ultrasonic wave device to determine the longitudinal modulus of elasticity, E
d
, a
representative number of boards were nondestructively tested, and the average value of E
d
was found to be
8,730 N/mm
2
(1.27x10
6
psi).




Figure 3. Beam specimen dimensions and loading.





5


Figure 4. Wood deck, steel reinforcement, and formwork

The concrete layer was a pre-mixed batch delivered with 24.15 MPa (3,500 psi) specified 28 day strength.
The concrete was consolidated by vibration and moist cured. A 70 N-m (50 lb-ft) torque was applied to all the
connector nuts after curing. A hydraulic ram was used to apply a center point load on the simply supported
span of 6.07 m (19’-11”). The deck was loaded from zero to 40 kips at a rate of 4 kips/min applied in five
cycles. Displacements were measured (using string potentiometers) at mid-span, quarter points, and at
the center of each notch to measure the relative slip between the wood and concrete layers.



Figure 5. Wood-concrete beam test specimens.


5.
RESULTS


Efficiency of the layered beams in developing composite action was determined using an established
definition (Pault and Gutkowski 1977).



6











=
)(
)(
100
CN
IN
DD
DD
Efficiency
(1)

Where
D
C
is the theoretical fully composite deflection,
D
N
is the theoretical fully non-composite
deflection, and
D
I
is the measured deflection for incomplete composite action of the specimen.

-1.44
-2.81
-4.01
-4.97
-5.60
-5.83
-5.60
-4.97
-4.01
-2.81
-1.44
0
-1.44
-2.81
-4.01
-4.97
-5.60
-5.83
-5.60
-4.97
-4.01
-2.81
-1.44
0
-0.78
-1.51
-2.18
-2.72
-3.09
-3.23
-3.09
-2.72
-2.18
-1.51
-0.78
0
-0.78
-1.51
-2.18
-2.72
-3.09
-3.23
-3.09
-2.72
-2.18
-1.51
-0.78
0
-0.37
-0.71
-1.02
-1.27
-1.45
-1.51
-1.45
-1.27
-1.02
-0.71
-0.37
0
-0.37
-0.71
-1.02
-1.27
-1.45
-1.51
-1.45
-1.27
-1.02
-0.71
-0.37
0
-177.9 3
-177.9 3
-177.9 3
Z
X
￿￿      ￿￿  ￿
   ￿  
 ￿ ￿ ￿ ￿  ￿ 
￿
 ￿  ￿ ￿  ￿ 
￿
 ￿   ￿ ￿  ￿ 
￿
     ￿  ￿ ￿   
    ￿    


Figure 5. Computed Deflected Shapes: Semi-Composite (top),
Non-Composite (middle), Full-Composite (Bottom)

Figure 5 presents deflections were computed using a commercial finite element model (Inter-CAD Ltd.,
1999). Table 1 compares the maximum measured and computed mid-span deflections for the two types
of notch connections: the glued-dowel connection, and the mechanical connection. The computed and
measured deflections are in good agreement, however the finite element model (FEM) shows slightly
higher stiffness. The non-composite model has a 5.84 cm mid-span deflection while the fully composite
model has 1.51 cm. Using Equation 1 the FEM results give an efficiency of approximately 60%. This is
within the range of values (55% - 77%) observed for beams in which the glued connection was used
(Brown 1998, Brown et al. 1998).

Table 1. Mid-span deflections for a point load of 177.93 kN at the mid-point

Computed
FEM value
Measured
(Dowel)
Measured
(Mechanical)
3.23 cm 3.53 cm 3.55 cm


A typical load-slip characteristic (horizontal, H, and vertical, V) measured for one cyclic loading is shown
in Fig. 6. Relative vertical and horizontal slip measurements were taken at the center of the notch by
string potentiometers mounted on brackets fixed rigidly to the bottom wood layer. The slip measurements
at the notch locations were used as input to develop the partially composite FEM of the beams.



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V1 & H1 vs. Load
0.000
20.000
40.000
60.000
80.000
100.000
120.000
140.000
160.000
180.000
200.000
-0.500 -0.450 -0.400 -0.350 -0.300 -0.250 -0.200 -0.150 -0.100 -0.050 0.000 0.050
Displacement (cm)
Load (KN)
V1
H1


Figure 6. Typical Load-Slip Characteristic of a Notch.


6.
CONCLUSIONS


The long-term objective of the initial research presented in this paper is to configure and load test full-
scale wood-concrete deck specimens sufficient in depth for use in short span bridges. Deck specimens
are to be constructed with two types of notched shear key/anchor connections such that a comparison of
the two connections can be made. In this initial study, preliminary beams are being examined. With
respect to the primary objectives it was found that a modest degree of composite action was developed in
the layered wood-concrete beams. A small to modest degree of improvement of efficiency would reduce
deflections under live load serviceability guidelines of bridge deck design specifications.

Table 1 shows the FEM was successful in predicting the response of the layered beams, thus
encouraging further use to be applied to deck specimens. Table 1 also shows a comparison of the two
connections types. The results can reasonably be interpreted that the modified fully extended shank
dowel appears comparable to the glued dowel. This could potentially be advantageous because the
modified dowel requires no adhesive and costs less than the original glued dowel. Further research on
creep deflections and durability under repetitive loads and extremes of temperature and humidity need to
be examined. Recently an extensive funded research program has been initiated to investigate these
aspects.


ACKNOWLEDGEMENTS

Initial work on the application to commercial and home construction was funded by the U.S. Department
of Agriculture through its National Research Initiative Competitive Grants Program (Grant 95083803).
The ongoing work on durability is also funded by that agency and program (Grant 2001-35504-10043).
The U. S. Department of Transportation provided support for studies for possible application in bridges.
That funding was via the Mountain Plains Consortium (MPC), which is federally sponsored through the
University Transportation Centers Program. The work conducted at CSU in the Structural Engineering
Laboratory of the Engineering Research Center whose facilities were provided as cost share to the
project. The contents reflect the views of the authors, who are responsible for the facts and accuracy of
the information presented herein. The U.S. Government assumes no liability for the contents or use
thereof.


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